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Aromatic substitution in the gas phase. Alkylation of arenes by C4H9+ ions from the protonation of C4 alkenes and cycloalkanes with gaseous Broensted acids.
No full textUse of a reactive aromatic substrate (o-xylene) to sample the isomeric population of the butyl ions from the gas-phase protonation of the C4H8 hydrocarbons shows that the primary product from linear olefins and methylcyclopropane is the s-butyl ion, or at least an s-butylating charged species displaying a positional selectivity very similar to that measured for the thermal s-butyl cations obtained from n-butane. A fraction of the primary C4H9+ ions isomerizes to the most stable tertiary structure, to an extent which depends, inter alia, on the exothermicity of the proton transfer process. Protonation of cyclobutane gives an alkylating reagent whose s-butylation/t-butylation ratio and positional selectivity set it apart from the C4H9+ reagents obtained from the other C4H8 hydrocarbons, suggesting the intervention of a different electrophile, conceivably protonated cyclobutane. Finally, protonation of isobutene yields exclusively a t-butyl ion that does not isomerize despite the large exothermicity of its formation
Gas-phase condensation of tert-butyl cations with Lewis bases
No full textt-Butyl cations, obtained in the dilute gas phase from the radiolysis of neopentane, react with H2O, MeOH, and EtOH giving high yields of the condensation products, t-butyl alcohol, t-butyl methyl ether, and t-butyl ethyl ether respectively; the relative rates of the condensations with H2O and MeOH, and of proton transfer to NH3, are very similar in gaseous neopentane at atmospheric pressure
Gas-phase reactions of arylium ion with hydrocarbons. 2. Gas-phase reactions of free phenylium cations with C3H6 hydrocarbons
No full textFree, unsolvated phenylium ions formed by the spontaneous β decay of a constituent atom of multitritiated benzene have been allowed to react with gaseous propene and cyclopropane in the pressure range from 10 to 700 torr. Phenylium ions attack efficiently both the C-H and the C-C cyclopropylbenzene and indan as the major products. Selective attack of phenylium ions on the π bond of propene is suggested by the compn. of tritiated products, isomeric phenylpropenes and isopropylbenzene. The different behavior of propene and cyclopropane toward gaseous phenylium ions is consistent with the results of related radiolytic investigations concerning gaseous systems at nearly atm. pressure. The reactivity pattern of the isomeric C3H6 hydrocarbons toward gaseous phenylium ions is discussed and compared with pertinent mass spectrometric data
Trapping with gaseous methanol the C7H7+ daughter ions from the decay of tritiated toluenes
No full textLabeled daughter ions from the decay of selectively multitritiated toluenes were trapped with MeOH(g) at different pressures. The ions from C6X5Me (X = H, T) yield tritiated Me tolyl ethers, while those from PhCX3 yield exclusively tritiated benzyl Me ether. The primary decay ions retain their cyclic structure, without collapsing to the tropylium ion, at least during the time (10-9 to 10-7 s) required for their trapping by the gaseous nucleophile
Predominant O-alkylation in the gas-phase attack of tert-butyl cations on phenol and anisole
No full textIn the gas-phase attack of t-C4H9+ ions, obtained in the dilute gas state from the radiolysis of neopentane, on phenol and anisole, the competition between n-type and π-type nucleophilic centres of the ambident substrates is strongly biased in favour of the oxygen atom, leading to the predominant formation of the t-butylated oxonium ions as the major reaction intermediates
Structure and reactivity of C7H7+ ions from the decay of tritiated toluenes. 1. Reactions of free tolyl ions with methanol in the gas and liquid phases
No full textLabeled tolyl cations from the decay of ring-multitritiated toluene have been allowed to react with methanol in the liq. and the gas phases, at pressures ranging from 6 to 100 torr, yielding Me tolyl ethers as the major products, without appreciable formation of benzyl Me ether. The isomeric compn. of the products from the gaseous systems depends on the pressure, the percentage of o-tolyl ether increasing at the expense of the para isomer as the methanol pressure is reduced. The results show that the three tolyl ions exist as distinct species in the dil. gas state. When formed in a sufficiently excited state, as from the β decay of a 3H atom in toluene, they undergo appreciable interconversion, without detectable isomerization to the benzyl cation, at least within the pressure range accessible to the decay technique
A radiolytic study on the gas-phase protonation of pinacol. The first demonstration of pinacol rearrangement in the dilute gas state.
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Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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